Updated: Sep 7, 2020
All the numbers associated with ISO ratings, shutter speeds and aperture values can be very confusing but, it can be simply explained as a balancing.
The relationship between shutter and aperture is relatively simple and linear.
As an example:
Your aperture is set to f/8 and your shutter is set to 1/15 lets say, you decide you want to change your shutter speed by two stops to 1/60 (faster).
By changing the shutter to a faster speed, you're letting in less light to the sensor. So, to keep the same exposure as before you'll need to change to a wider aperture of f/4 which is also two stops.
So in summary
f/8 and 1/15 = f/4 and 1/60
The ISO rating is similar in operation, in that doubling or halving is equal to one stop (e.g. ISO 200 to ISO 400 or ISO 200 to ISO 100). The ISO setting does not affect the Shutter Speed or Aperture setting at all, all it does is tinker with the sensitivity of the senor. So in bright light conditions (a sunny day), you need a sensor that isn't so sensitive to the bright light (i.e. ISO 100) and in darker conditions, like an overcast day, ISO should be set to a more sensitive setting (i.e ISO 400)
As I mentioned earlier, its all about the balance between ISO, Aperture and Shutter and this is called the "Exposure Triangle".
The Exposure Triangle is all about the balancing of these three things. The quicker the shutter speed is set, the more you reduce the amount of time the light hits the sensor. And so because of this, the more you will need to add extra light to the sensor by opening up the aperture more. Finally, use the ISO rating to fine tune the balance.
Personally, when I set up my camera in manual, I think about aperture first. What DOF (Depth of Field) am I looking for? Shallow (e.g. f/2.8 or perhaps f/4) which is great for macro shots or a broad DOF (e.g. f/11 or f/16) which is great for landscapes.
I then figure out shutter speeds from there based on the amount of atmospheric light that is available.
And then finally I use ISO to fine tune the balance. Try it for yourself and show me your results.
I hope you have enjoyed this brief explanation, if you have any questions please do make a comment below.
In my next post, I will be explaining light metering and 18% grey cards.